DNA is, as you say, "unzipped" is a complex process. DNA usually unwinds either to replicate DNA or synthesize mRNA to make proteins. I'll use DNA replication as the example. …when the process starts, it will start at hundreds of sites along the length of the DNA molecule. once started the replication will proceed in both directions. DNA helicase is the enzyme that opens the two strands, and the enzyme topoisomerase is before helicase to make sure no tension builds up as the strands unwind. after topoisomerase runs down the length, closely followed by helicase, binding proteins stick to the newly opened nucleotides to prevent the strands from sticking together. that's the first basic step of DNA replication and basically answers your question. Helicase opens it, but remember it must be triggered by a enzyme of protein in order for it to start, and then continues until the molecule is finished.

This method is called " Semi-conservative " which replicates the DNA so it begins with unzipping, unwinding the DNA by breaking down the hydrogen bonds between the nitrogenou…s bases with the help of " helicase enzyme " that helps separating the strands. After that the free nucleotides will be activated by adding extra two phosphate groups from ATP to provide Energy for binding, and then the activated nucleotides will attach to the DNA as that Adenine link with thymine with double hydrogen bonds and Cytosine link with Guanine with triple hydrogen bonds. And Finally the DNA polymerase enzyme will catalyzes the joining of the nucleotides of the new strands with the formation of phosphodiester bonds while the extra phosphate groups that were attached to the activated nucleotides are released.